1. Field of the Invention
The present invention relates to a driving circuit for an electroluminescence device and especially relates to a driving circuit in which the withstand voltage of an electroluminescence device can be improved to obtain superior luminescence with high brightness.
2. Description of the Related Art
A conventional electroluminescence device luminesces utilizing the luminous phenomenon caused by applying an electric field to a fluorescent base material such as zinc sulfide containing manganese or the like as a luminescence center.
A transparent type electroluminescence device is typically constructed with a transparent electrode made of indium tin oxide film (IT0) formed on a surface of a glass substrate by deposition. An insulating layer, a luminescence layer, another insulating layer, and a transparent electrode are formed on a surface of the ITO transparent electrode in turn by deposition.
When making a display panel 1 comprising seven segments utilizing electroluminescence devices as shown in FIG. 1, an electrode 11 arranged on one of the surfaces of an electroluminescence layer 13 is usually used as a common electrode in order to simplify the panel construction.
On the other hand, a plurality of other electrodes 12 are arranged on the surfaces of the electroluminescence layers opposite to the surface to which the electrode 11 is provided. Each of them serves as a driving electrode.
An equivalent circuit of an actual circuit of such an electroluminescence display panel 1 is shown in FIG. 2.
In the FIG. 2, the electroluminescence layers 13 existing between the common electrode 11 and each one of the driving electrodes 12 are capacitive load indicated as Ca to Cg.
The common electrode 11 is grounded through a wiring resistor RO.
Each driving electrode, i.e., segment electrode 12, is oppositely arranged to the common electrode 11 with one of the electroluminescence layers 13 (loads Ca to Cg) interposed therebetween and is connected to a driving circuit (not shown) through one of the wiring resistors Ra to Rg.
FIG. 2 shows a condition where the electroluminescence devices are driven. In this condition, each electroluminescence device is driven to luminesce with a positive voltage +V and a negative voltage -V which are intermittently and alternately applied to each of the driving electrodes.
Since the circuit constants of the driving circuits differ from each other, the phases of the driving voltages Vb and Vc applied to the electroluminescence devices rarely are completely synchronized with each other.
If the driving voltage Vb is delayed in phase as shown in FIG. 2, it remains at a certain level when the driving voltage Vc falls to 0 V.
FIG. 2 shows a condition where the driving voltage Vc rises from a negative voltage -V to 0 V. In this situation, an electric charge +Q in a surface of the electroluminescence layer 13 on which the common electrode 11 is arranged is discharged to other electroluminescence devices through the common electrode 11.
Since the resistivity of ITO film, frequently used for the transparent electrode, is extremely large compared with that of aluminum film or the like, the resistance of the resistor Ro is increased, so the potential of the common electrode 11 is increased, causing a spiked voltage exceeding the driving voltage in an electroluminescence device which is charged and luminesces.
For example, when the resistance of the resistor Ro is infinity, the spiked voltage generated in a circuit when a phase of only one of signals Va to Vg is advanced from phases of another signals, as shown in FIG. 2 becomes around eight-sevenths the driving voltage.
Therefore, a problem arises in that the level of the spiked voltage as explained above will be increased when all but one of electroluminescence devices are simultaneously discharged and extinguished. This leads to breakage of the electroluminescence devices.
This kind of problem will occur when the driving voltages are delayed in phase from each other and when switching the driving operation of the electroluminescence devices due to a display change.
From this point of view, it is conventionally required that a level of a rated driving voltage of an electroluminescence display panel be sufficiently reduced when the resistance of the resistor Ro is large.
Therefore, a problem arises in that a luminescent display having a high level of brightness cannot be obtained.